Question

Draw a labelled diagram of Van de Graaff generator. State its working principle to show how by introducing a small charged sphere into a larger sphere, a large amount of charge can be transferred to the outer sphere. State the use of this machine and also point out its limitations.

Answer 1

Van de Graaff generator is a device used for building up high potential differences of the order of a few million volts.

Principle − It is based on the principle that charge given a hollow conductor is transferred to the outer surface and is distributed uniformly over it.

Construction:

It consists of a large spherical conducting shell (S) supported over the insulating pillars. A long narrow belt of insulating material is wound around two pulleys P₁ and P₂. B₁ and B₂ are two sharply pointed metal combs. B₁ is called the spray comb and B₂ is called the collecting comb.

Working − The spray comb is given a positive potential by a high tension source. The positive charge gets sprayed on the belt.
As the belt moves and reaches the sphere, a negative charge is induced on the sharp ends of the collecting comb B₂ and an equal positive charge is induced on the farther end of B₂.
This positive charge shifts immediately to the outer surface of S. Due to discharging action of sharp points of B₂, the positive charge on the belt is neutralised. The uncharged belt returns downwards and collects the positive charge from B₁, which in turn is collected by B₂. This process is repeated and the positive charge on S goes on accumulating. In this way, voltage differences of as much as 6 or 8 million volts (with respect to the ground) can be built up

Use: Van de Graaff generator generates high potential differences that are used to accelerate charged particles such as electrons, protons, ions, etc. used for nuclear disintegration.

Limitations: 1) It's a series combination that allows only one route for the movement of charge.

2) It can accelerate only the charged particles not the uncharged particles.